1. Technical Field of the Invention
The present invention relates to memories made as an integrated circuit and, more particularly, to those which include column redundancy circuits. The invention more particularly, but not exclusively, applies to so-called toggle Magnetoresistance Random Access Memories (MRAM).
2. Description of Related Art
An integrated circuit memory includes many memory cells arranged in rows and columns on a very small surface. Each memory cell is then selectable by a word line associated with each row and a bit line associated with each column.
Methods for manufacturing integrated circuit memories very often introduce defects, notably those which affect the memory cells. Also, in order to avoid a too large number of rejects and with a concern to improve manufacturing yields, the manufacturers of these circuits provide a certain number of so-called redundant cells intended to replace the defective cells of the memory if need be.
In practice, for reasons of simplicity, replacement of a defective memory cell is achieved by replacing the totality of the row or of the column including this cell. The memory element, either row or column, which should be replaced, will depend on the type of detected defect. However, within the scope of the present invention, one is only interested in the redundancy of columns. It is therefore common to associate with the memory scheme, redundancy columns intended to be substituted for columns of the memory plane including one or more defects.
For memories of large capacity intended to provide data as n bit words, provision is made for the number of columns to be much larger than n, and, in this case, the columns are arranged in n subspaces of columns each including p columns of cells. Finally, each column comprises m cells, i.e., the memory includes m rows. By considering the aspect of reading data contained in the memory, each column subspace of the memory will provide one bit of the word at the output of the memory, by collecting at each read-out a datum from a determined column of the relevant subspace. Means for column decoding are then provided for each subspace of the memory, intended to select one column from p columns in the relevant subspace.
Column redundancy means, as at least a replacement bit line, are typically provided within each subspace of the memory, with which means a defective bit line from the p bit lines of the subset may be compensated. The stake then consists of enabling these redundancy means within each subspace at the right moment. More specifically, when a bit line is recognized as defective, the memory should be able to prevent its selection to the benefit of selecting the replacement bit line within the relevant column subspace.
For this purpose, the column redundancy means are put into service as a result of tests carried out before making each integrated circuit, with which all the defective bit lines may be listed. Programmable reconfiguration means are then provided in the memory at each column subspace, so that when the test has revealed a faulty element, the latter is automatically replaced with a replacement element selected from the redundant elements, this replacement should be invisible for the user and without any consequences on the performance of the memory. Thus, for the user, substitution of the columns within the memory should be transparent, i.e., he/she is allowed to send to the input for addressing the memory, an address involving a defective column. In practice, these reconfiguration means apply circuits for detecting whether the current address presented to the memory corresponds to that of a defective column and if this is the case, for selecting a redundant column as a replacement for the faulty column.
This automatic replacement is usually carried out by means of a register provided for locally storing a code of the addresses of the defective columns, downloaded beforehand into the memory. The redundancy columns may then be selected by a circuit for enabling redundancy, which essentially comprises decoding means capable of decoding the code of defective addresses locally stored, and of comparing the addresses of columns received by the memory with the addresses of defective columns decoded beforehand. In the case when a defective column address is applied to the memory, it is therefore recognized by the redundancy enabling circuit, the role of which is then to inhibit the corresponding column decoder and to select a redundancy column as a replacement for the defective column. More specifically, in a given subset of columns of the memory, the defective column is definitively inhibited and the redundancy column is definitively enabled in its place.
This state of the art typically corresponds to the redundancy configuration selected for DRAM type memories, for example. Thus, in this context, and as stated above, the column redundancy means are incorporated inside each subspace of columns of the memory, with however the drawback of having to integrate into the memory specific means for enabling these redundancy means, as an adjunct to standard elements such as read and write circuits and address decoders. These additional means which have to be integrated into the memory for managing redundancy, are formed by the local storage means for the code of addresses of defective columns, and their associated decoding means, which deliver a signal for enabling the redundancy, allowing automatic selection of the associated redundancy column and, in parallel, inhibition of selection of the defective column.
Such an architecture for enabling redundancy means inevitably causes a substantial increase in circuit bulkiness (size or area), not only at the level of the actual memory but also at the level of the peripheral circuitry where the circuit density is already very large. Considering the limited amount of space which is generally available in the integrated circuit memories, it would then be advantageous to be able to omit these additional means in order to activate column redundancy means of the memory.
There is a need in the art to find a remedy to the aforementioned drawbacks. It would be advantageous if, with respect to a column redundancy enabling system in an integrated circuit memory, access to the redundant elements did not require provision of means specifically dedicated for this purpose in the memory.